meiosis and genetic diversity

ploidy levels

refers to chromosome set number

haploid (1N)

one chromosome set

dipoid

two chromosome sets

triploid

three chromosome sets

homologous chromosomes

• paired chromosomes from maternal and paternal sources

• similar length and gene content

• same genes, different alleles

meiosis

• sexual reproduction

• produces 4 haploid gamate cells from one diploid parent cell

sexual reproduction

haploid gamete from each parent fuse to generate diploid offspring

importance of meiosis

ensure gamete cells are haploid

meiosis ( process)

• process which chromosome number is reduced by half

  • diploid → haploid

• only for gamete cells (specialized haploid cells)

• involves 2 nuclear divisions

  1. meiosis i - reduction division

     • diploid → haploid,

  2. meiosis ii - equational division

     • separation of chromatids ( like in mitosis)

meiosis : reduction division

• reduction of chromosome number from diploid to haploid

  • exchange of genetic information,

  • results in 2 haploid cells

• includes

  1. prophase I - homologous chromosomes pair via synapsis, crossing over occurs

  2. metaphase I - homologous chromosomes align @ equator

  3. anaphase I - homologous chromosome seperation

  4. telophase I & cytokineses - cell divides to form 2 cells

meiosis i : prophase

• bivalents form, chromosomes pair up

• crossing over occurs between adjacent non sister chromatids

  • chiasma visible, synaptonemal complex dissapears

• nuclear envolope fragments

describe steps leading to synaptonemal complex dissapearing

1. chromosomes condense

2. bivalent formation, synaptonemal complex forming

3. synapsis occurs, crossing over begins

4. crossing over occurs, chiasmata visible

5. synaptonemal complex disapears

6. bivalent ready for metaphase I

meisois i - metaphase i

bivalents ( paired chromosome pairs) move and line up at equator

meiosis i - anaphase i

sister chromatids ( chromatids) move towards the spindle poles

• each spindle pole carries one set of chomosomes ( haploid)

meiosis i : telophase i and cytokinesis

telophase i - when chromosomes reach spindle poles

• formation of nuclear envolopes at spindle poles

• formation of cleave furrow

• cytokinesis,

generates 2 haploid cells, each cell contains chromosomes that has two sister chromatids

exchange of genetic information

• 2 ways

  1. independent assortment

  2. crossing over

independent assortment

chromosomes distribute randomly

• in humans 2²³ chromsome combinations possible

crossing over

• requires homologous chromosomes to be close togethr

• physical exchange of genetic information

  • creates new genetic combinations

gametogenisis

generation of either male of female gamete cells

gametogenisis of sperm cells

a diploid spermatocyte → 4 haploid spermatids → 4x sperm cells

gametogenisis of eggcells

• a diploid oocyte → 3 polar bodies + 1 ovum

differences with mitosis and meiosis stages

meiosis ii - equation division

occurs in all 4 stages

sister chromatid seperation

produces 4 genetically different haploid cells

outcome of meioisis

• production of 4 genetically differing haploid cells

defects in meiosis

1. nondisjunction ( chromosome number )

2. chromosome structure

nondisjunction

• chromosome partitioning error

  • causes aneuploidy - abnormal chromosome number

    eg down syndrome

defects mitosis : structural

• deletions

• duplications

• translocations

• diversions

• potential cancer associations